:root {
    --primary-color: #007bff;
    --secondary-color: #f8f9fa;
    --text-color: #343a40;
    --sub-text-color: #6c757d;
    --bg-color: #f8f9fa;
    --white-color: #ffffff;
    --shadow-color: rgba(0, 0, 0, 0.1);
}

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body {
    font-family: 'Roboto', sans-serif;
    font-size: 16px;
    line-height: 1.6;
    color: var(--text-color);
    background: var(--bg-color);
    padding: 20px;
    display: flex;
    justify-content: space-between;
}

.content,
.course-menu {
    border-radius: 12px;
    box-shadow: 0 4px 8px var(--shadow-color);
    padding: 20px;
}

.content {
    flex: 1;
    margin-left: 20px;
    background-color: var(--white-color);
}

.course-menu {
    width: 300px;
    background: var(--white-color);
    padding: 20px;
    float: right;
    margin-right: 20px;
    margin-bottom: 20px;
}

.menu-title {
    font-size: 20px;
    font-weight: bold;
    margin-bottom: 15px;
    color: var(--primary-color);
    text-align: center;
    padding: 10px;
    border-bottom: 2px solid var(--primary-color);
}

.chapter {
    margin-bottom: 15px;
}

.chapter-title {
    font-size: 16px;
    display: block;
    padding: 12px;
    margin-bottom: 10px;
    border-radius: 8px;
    cursor: pointer;
    transition: background 0.3s, transform 0.2s;
    background: white;
    box-shadow: 0 2px 4px rgba(0, 0, 0, 0.05);
}

.chapter-title:hover {
    background: var(--secondary-color);
    transform: scale(1.02);
}

.chapter-title.active {
    background: #1890ff;
    color: white;
    box-shadow: 0 4px 8px rgba(0, 0, 0, 0.15);
}

.chapter-title:active {
    transform: scale(0.98);
}

.section-list {
    margin-left: 25px;
    display: none;
    transition: max-height 0.3s ease-in-out;
    overflow: hidden;
}

.chapter.expanded.section-list {
    display: block;
}

.section {
    padding: 8px;
    cursor: pointer;
    color: var(--sub-text-color);
    font-size: 14px;
}

.section:hover {
    color: var(--primary-color);
}

.video-container {
    width: 100%;
    max-width: 800px;
    margin: 20px auto;
    background: #000;
    border-radius: 8px;
    overflow: hidden;
    border: 1px solid #ccc;
    box-shadow: 0 0 10px rgba(0, 0, 0, 0.1) inset;
}

.video-container iframe {
    width: 100%;
    aspect-ratio: 16/9;
    border: none;
}

.course-intro {
    color: var(--sub-text-color);
    line-height: 1.8;
    margin-top: 20px;
    background: var(--white-color);
    padding: 20px;
    border-radius: 8px;
    box-shadow: 0 2px 8px var(--shadow-color);
}

.objectives {
    margin-top: 20px;
}

.objectives h3 {
    font-size: 16px;
    margin-bottom: 10px;
    color: var(--text-color);
}

.objectives ul {
    list-style: none;
    padding-left: 20px;
}

.objectives li {
    margin-bottom: 8px;
    color: var(--sub-text-color);
    position: relative;
}

.objectives li:before {
    content: "•";
    color: var(--primary-color);
    position: absolute;
    left: -15px;
}

.loading-icon {
    display: flex;
    align-items: center;
    justify-content: center;
    height: 100%;
    background: rgba(0, 0, 0, 0.5);
    border-radius: 8px;
}

.loading-icon::before {
    content: "";
    width: 40px;
    height: 40px;
    border-radius: 50%;
    border: 4px solid #fff;
    border-top-color: #1890ff;
    animation: spin 1s linear infinite;
}

@keyframes spin {
    0% {
        transform: rotate(0deg);
    }

    100% {
        transform: rotate(360deg);
    }
}